The present invention relates to the selective production of citric acid and more specifically to the production of citric acid by fermentation of a mutant yeast strain which requires the presence of iron within the culture medium.
Citric acid has wide uses and is useful for example, for the preparation of beverages and pharmaceutical syrups.
Heretofore, there have been proposed in the art various processes for the production of citric acid using yeasts. When citric acid is produced by fermentation using yeasts, usually, a considerable amount of isocitric acid occurs in the culture medium as a by-product. Isocitric acid is an organic acid for which there is not a significant amount of commercial use. Thus, it is rarely desired to produce isocitric acid. Moreover, the by-production of isocitric acid renders isolation and purification of citric acid from the culture medium very complicated.
On the other hand, it is known that the production of isocitric acid in a fermentation process is accelerated by the presence of iron ion or iron-containing ion such as Fe(CN).sub.6.sup.-4 [Journal of the Agricultural Chemical Society of Japan, Vol. 44, p. 562 (1970), Japan and U.S. Pat. No. 3,773,620]. Therefore, in order to prevent the by-production of isocitric acid, it is necessary that iron ion or iron-containing ion be absent from the medium. However, from a commercial standpoint, contamination of the medium by iron ion is unavoidable due to the fermentation raw materials and equipment. For example, in the commercial production of citric acid, usually an apparatus made of stainless steel or one having a glass lining is used. In either case, contamination of the medium with iron ion is unavoidable. Even the slight contamination of the medium due to the apparatus is considered to be sufficient to cause a significant production of isocitric acid and, therefore, renders the process impractical.
U.S. Pat. No. 3,689,359 discloses that a citric acid-accumulating yeast belonging to the genus Candida which is incapable of utilizing citric acid suppresses production of isocitric acid while producing a high yield of citric acid. However, this process also suffers from serious drawbacks, and a more practical method is still in demand.
Accordingly, the present invention is concerned with the suppression of the by-production of isocitric acid in a fermentation process while maintaining the production of citric acid in a yield equal to or greater than that of the same process with the by-production of isocitric acid. In such manner, a commercially feasible process is attained which substantially eliminates the heretofore necessary step of purification of the citric acid.